Simplify the following expression: $a = \dfrac{1}{3t} - \dfrac{1}{9t}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $3t$ and $9t$ $\lcm(3t, 9t) = 9t$ $ a = \dfrac{3}{3} \cdot \dfrac{1}{3t} - \dfrac{1}{1} \cdot \dfrac{1}{9t} $ $a = \dfrac{3}{9t} - \dfrac{1}{9t}$ $a = \dfrac{3 -1}{9t}$ $a = \dfrac{2}{9t}$